The inventor previously found novel naphthyridine derivatives that have an analgesic effect, and accomplished an invention relating to the derivatives [PCT/JP98/03045 (international publication No. WO 99/02527A1)]. However, in continued research, the inventor found that the derivatives show little effect in the case of treating diseases, such as diabetic neuropathy, in which the nervous system is damaged, barely achieving the desired analgesic effect, and furthermore, the inventor recognized that administration of the derivatives may cause side effects, such as hypertrophy of the thyroid gland, etc.